Light-emitting diodes (LEDs), particularly including organic light emitting diodes (OLEDs), are of great interest for both display and lighting applications. However, due to the high refractive index of OLED materials, reduced light outputs caused by light trapping within either the OLED materials or other material layers within the devices remain a significant problem.
A number of light-scattering approaches for improving the extraction of light from LED and OLED devices have been proposed. These include, in addition to the use of roughened light scattering surface layers, resin covering layers containing light-scattering inclusions, bonded ceramic covering layers of light-scattering particles, and layers formed of opal or other light-scattering glasses. Each of these proposed solutions presents its own problems, however, including increased materials costs, limited layer durability, and/or added processing complexity.
Opal glasses typically comprise one or more light scattering phases evenly dispersed in a fully encapsulating glass matrix. It was thought that this combination of features could provide an impermeable, durable and stable light scattering material for OLED applications. However such glasses have not found substantial commercial use for these applications due to a number of actual and potential problems. Significant shortcomings of conventional opal glasses include, for example, a relatively low refractive index absent the use of expensive, index-increasing glass modifiers, a need in some cases to use supplemental heat treatments to develop a useful level of light scattering at low glass thicknesses, and undesirable light attenuation by the matrix glass or light-scattering phase(s) in glass layers of higher thickness.